Method of providing a metal substrate with a metallic coating

ABSTRACT

THE METHOD OF PROVIDING A SURFACE OF A METAL SUBSTRATE WITH A METALLIC COATING COMPRISES CLEANING THE SAID SURFACE BY REMOVING ANY METALLIC OXIDE OR OTHER CONTAMINATION THEREFROM WITHOUT THEREBY INTRODUCING HYDROGEN INTO THE SAID SURFACE, COATING THE CLEANED SURFACE WITH BOTH A SOLUTION OF SODIUM OR POTASSIUM SILICATE AND WITH A METALLIC POWDER, AND SUBSEQUENTLY TREATING THE COATED SUBSTRATE TO MAKE THE METALLIC COATING ADHERE TIGHTLY THERETO.

Feb. 23, 1971 p CQSTELLOE ETAL METHOD OF PROVIDING A METAL SUBSTRATE WITH A METALLIC COATING Filed May 13, 1968 United States Patent 3,565,663 METHOD OF PROVIDING A METAL SUBSTRATE WITH A METALLIC COATING Patrick Costelloe, Chester, and Albert Edward Jackson, Wendover, Gwernafiield, Mold, England, assignors to John Summers & Sons Limited, Flintshire, England, a British company Filed May 13, 1968, Ser. No. 728,440 Claims priority, application Great Britain, June 8, 1967, 26,584/67 Int. Cl. C23c 17/00 US. Cl. 117-17 12 Claims ABSTRACT OF THE DISCLOSURE The method of providing a surface of a metal substrate with a metallic coating comprises cleaning the said surface by removing any metallic oxide or other contamination therefrom without thereby introducing hydrogen into the said surface, coating the cleaned surface with both a solution of sodium or potassium silicate and with a metallic powder, and subsequently treating the coated substrate to make the metallic coating adhere tightly thereto.

This invention concerns a method of providing a metal strip, such, for example as a steel strip, with a metallic coating, such as a coating of aluminium.

According to the present invention, there is provided a method of providing a surface of a metal strip with a metallic coating comprising passing the strip through a dilute nitric acid bath to remove metallic oxide from said surface without thereby introducing hydrogen into the said surface, coating the cleaned surface during movement thereof with both a solution of sodium or potassium silicate and with a metallic powder which is bondable to the metal strip by a metal to metal bond, drying the coated strip, rolling the strip to compact the coating, and then heat treating the coated strip by raising it to an elevated temperature at which a metal to metal bond between the strip and the metallic coating is produced.

The said solution preferably contains from 1.0 to 5.0 grams of potassium or sodium silicate per liter of water. From 0.75 to 2.5 cubic centimeters of the said solution is preferably applied to each square foot of the said surface.

Since any metallic oxide will have been removed from the said surface of the strip, and since oxide skin on the metallic powder will be broken during compaction of the coating, the said compaction and subsequent heat treatment will result in a metal to metal bond between the strip and the metallic coating, the sodium or potassium silicate binding the metallic powder to the strip when the liquid in the said solution has been evaporated by the said drying.

It would be possible, in contrast to the present invention, to bond the strip to the metallic coating by providing each of them with an oxide film, and bonding the oxide films together. The said metal to metal bond is, however, to be preferred because it is difficult to ensure that such oxide films are really properly formed and therefore difficult, Where such oxide films are used, to prevent subsequent delamination of the coating. Moreover, the formation of such oxide films requires heating in air and therefore necessitates rolling the substrate while hot. This, however, is not satisfactory, since the coating is liable to become transferred to the rollers which effect the said rolling.

For this reason, except during a drying or heat treating stage, all the stages of the method of the present invention are preferably effected at room temperature.

The strip is pickled in a dilute nitric acid bath, since nitric acid is the only common acid which does not introduce hydrogen into the said surface during pickling. The said bath preferably contains material, e.g. urea, which inhibits the accumulation of nitrous acid and oxides of nitrogen therein, since nitrous acid and oxides of nitrogen, if allowed to accumulate, render the bath inactive. Urea, moreover, inhibits the accumulation in the bath of corrosion products of steel with nitrous acid and oxides of nitrogen.

In one form of the present invention, the strip is wetted with the said solution, and the powder is subsequently applied thereto in an electrostatically charged condition.

In another form of the present invention, a slurry is formed of the said solution and powder, and this slurry is applied to the strip. This slurry may be applied to the strip by a curtain coater. Alternatively, the slurry may be applied to the strip by a roller coater, the slurry having previously been rendered thixotropic.

The invention also comprises a metal strip when coated by the method set forth above.

The invention is illustrated, merely by way of example, in the accompanying diagrammatic drawing, which is an elevation of an apparatus for carrying out the method of the present invention.

In the drawing, there is shown an apparatus comprising an uncoiler 1 which carries a roll 2 of mild steel strip 3. The strip 3 which is uncoiled from the roll 2 passes over two spaced apart guide tables 4, 5. Between the tables 4, 5 there is disposed a shear 6 which may be operated, by means not shown, to shear the strip 3 as required.

The strip 3 passes through apparatus 7 for applying a back tension to the strip 3 and thence along a cleaning line (not shown) which comprises a degreasing bath, where the strip is subjected to a degreasing liquid while being scrubbed, a cold water rinse, a pickling bath, a further cold water rinse, and a drying plant.

The pickling bath contains dilute nitric acid, e.g. it may contain 5% by weight nitric acid, and in addition contains 25 grams per litre of urea. The latter inhibits the accumulation in the bath of nitrous acid and oxides of nitrogen which, if allowed to accumulate, would render the bath inactive.

All the surface area of the strip 3 is thus cleaned in the cleaning line by having any metallic oxide or other contamination removed therefrom. At the same time, the use of a nitric acid bath effects a pickle which does not introduce hydrogen into the surface of the strip. It would be very undesirable for hydrogen to be introduced since it is readily expelled on heating and thus renders the metallic coating, which is subsequently applied to the strip, liable to delamination.

The strip 3 then passes over rollers 10, 11, 12 and 13 so as to pass beneath spray equipment 14. The spray equipment 14 sprays onto the adjacent surface of the strip 3 a solution containing sodium silicate e.g. in a concentration of 1.0 to 5.0 (and preferably of 3) grams of sodium silicate per litre of water. The sodium silicate may be a commercial sodium silicate containing 18% w./w. sodium oxide, and 36% w./w. of silica.

Alternatively, the sodium silicate may be replaced by a similar quantity of potassium silicate.

The spray equipment 14 subjects the adjacent surface of the strip 3 to the said solution in an amount from 0.75 to 2.5, and preferably about 1.5 cubic centimetres of the said solution for each square foot of the adjacent surface. This amount of solution is such as to trap the powder which is subsequently sprayed onto the strip while being insuflicient to cause pools of the solution to build up on the strip.

The strip 3 then passes beneath two powder deposition units 15. Aluminium powder, conveyed in a stream of nitrogen, is passed from meter rolls, which form part of the powder deposition units 15, and onto the adjacent surface of the strip 3. A voltage (of approximately 20,000 volts for each inch of distance between the bottom of the screens and the strip) is applied to the screens.

The aluminium powder thus falls in an electrostatically charged condition onto the adjacent surface of the strip 3 where it would normally lose its charge and be reattracted back upwardly were it not trapped by the solution on the strip. The potassium or sodium silicate, moreover, prevents oxidation or other corrosion of the aluminium powder by the water in the said solution.

The strip 3 which has been so coated then passes over rolls 16, 17 so as to pass beneath a second spray equipment 20 and then beneath two further powder deposition units 21, the parts 20, 21 having the same construction as the parts 14, 15 respectively. As will be appreciated, the spray and powder from the parts 20, 21 respectively will fall onto the opposite side of the strip 3 to that which has been coated with the spray and powder from the parts 14, 15.

The strip then passes through a high frequenoy heater 22, where it is dried, the sodium silicate, remaining on the strip acting as a binder which prevents the aluminium powder from falling off the strip, e.g. due to wall vibration. After this, the strip passes through a rolling mill 23 having rollers 24, 25 which compact the aluminium coating to the strip. The strip 3 is then coiled onto a roll 26 of a coiler 27 from which it is removed to be heat treated at an elevated temperature, to produce a metal to metal bond between the strip and the aluminium powder, e.g. by being heated at 500 C. for one hour, or by being heated at 350 C. for 15 hours.

In the apparatus described above, use is made of spray equipments 14, 20 and powder deposition units 15, 21. These parts, however, may be dispensed with, coating being effected by employing a slurry of aluminium powder in an aqueous sodium or potassium silicate solution, this coating being applied to the strip 3 by means of a curtain coater (not shown). The sodium or potassium silicate prevents corrosion of the aluminium powder and at the same time acts as an efiicient binder when the water of the slurry is subsequently evaporated in the heater 22. A typical slurry could contain about 150 grams of aluminium powder to 100 cc. of water containing about 0.5% w./w. of sodium silicate.

Alternatively, instead of using such a curtain coater, the slurry may be applied by using a reverse roller coater. In this case, the slurry would have to be rendered thixotropic. This could be achieved by increasing the concentration of aluminium powder in the slurry to about 500 to 600 grams per 100 mls. and by the addition of a small quantity of an immiscible hydrocarbon, for example xylol, plus a little alcohol, for example butanol. In this case, also, the sodium silicate or potassium silicate will prevent corrosion of the powder.

We claim:

1. A method of providing a surface of a metal strip with a metallic coating comprising passing the strip 0 through a dilute nitric acid bath to remove metallic 0X- ide from said surface without thereby introducing hydrogen into the said surface, coating the cleaned surface during movement thereof with both a solution of sodium or potassium silicate and with a metallic powder which is bondable to the metal strip by a metal to metal bond, drying the coated strip, rolling the strip to compact the coating, and then heat treating the coated strip by raising it to an elevated temperature at which a metal to metal bond between the strip and the metallic coating is produced.

2. A method as claimed inclairn 1 in which the said solution contains from 1.0 to 5.0 grams of potassium or sodium silicate per litre of water.

3. A method as claimed in claim 1 in which from 0.75 to 2.5 cubic centimetres of the said solution is applied to each square foot of the said surface.

4. A method as claimed in claim 1 in which, except during a drying and a heat treating stage, all the stages of the method are effected at room temperature.

5. A method as claimed in claim 1 in which the said bath contains material which inhibits the accumulation of nitrous acid and oxides of nitrogen therein.

6. A method as claimed in claim 5 in Which the material is urea.

7. A method as claimed in claim 1 in which the substrate is wetted with the said solution, and the powder is subsequently applied thereto in an electrostatically charged condition;

8. A method as claimed in claim 1 in which a slurry is formed of the said solution and powder, and this slurry is applied to the strip.

9. A method as claimed in claim 8 in which the slurry is applied to the strip by a curtain coater.

' 10. A method as claimed in claim 8 in which the slurry is applied to the strip by a roller coater, the slurry having been previously rendered thixotropic.

11. A method as claimed in claim 1 in which the metallic powder is aluminium powder.

12. A method as claimed in claim 1 in which the metal strip is a steel strip.

References Cited UNITED STATES PATENTS 2,132,511 10/1938 Hentrich 252101 2,177,751 10/ 1939 Sikorski 252-794 2,462,763 2/1949 Nightingall 117--135.1X 2,906,649 9/1959 Keuth et al. 117169X 2,944,919 7/1960 Morris et a1. 117135.1X 3,202,531 8/1965 Dowd 117135.1X 3,258,346 6/1966 Fisher 117135.1X 3,287,142 11/1966 Russell 117135.1X 3,382,085 5/1968 Wren et a1. 11717 3,423,229 1/1969 Kompanck et a1. 117-135.1X 3,461,840 8/1969 Turner 117-l05.3X 3,428,472 2/1969 Shimose et a1 11722 3,485,654 12/1969 McGraw et a1. 11765.2X 3,503,775 3/1970 Austin 11765.2X

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

